1. Field of the Invention
The present invention pertains to a high quality printing method utilizing a dot matrix serial printer.
2. Description of the Prior Art
In data processing systems and communication systems fast dot matrix printers are widely used. In such printers a print head provided with a plurality of printing needles, arranged in a vertical column, slides transversely on a printing support. The selective and timed actuation of the needles during the head movement causes the printing of characters by actuating pre-established needles of the matrix. Generally printing heads with 7 and 9 needles per line and 5 or 9 needles per column are used. These generate characters comprised of dots by causing selected needles to impinge on a paper to form a desired character. However the quality of these characters is limited by the number of the discrete dots which are utilized in the matrix. Nevertheless this technique provides high printing speed. Accordingly it is desirable to improve the technique so that the print quality is equivalent to that obtained by using solid fonts. A typical solution to this problem is disclosed in U.S. Pat. No. 4,159,882. The specification of this patent describes a conventional head having 7 or 9 needles vertically arranged with a pitch p between the centre of two contiguous needles (the needle diameter being slightly smaller than p). The printing of characters in a line is performed with several printing passes in order to increase the vertical resolution and to obtain vertical lines where the print dots overlap. Between one print pass and a subsequent one the printing support is vertically advanced by a submultiple of pitch p; i.e., 1/2 or 1/4. In this manner characters comprised of dots arranged according to a 28 line matrix can be obtained with a four pass printing by using a 7 needle head. The horizontal resolution of the matrix is increased by means of electronic circuits which enable the actuation of the needles in correspondence to subsequent printing positions. These printing positions occur at distances smaller than the diameter of the print head needles. This solution requires complicated mechanical devices and is expensive.
There are several problems that need attacking to obtain a cheaper and less expensive device. Because the pitch p between contiguous needles is 1/72" (0.35 mm) and the needles have a slightly smaller diameter (0.33 mm) than the pitch very complicated and precise equipment is necessary to advance the printing support by a half or a quarter of 0.35 mm, i.e 1/144" (0.175 mm) or 1/288" (0.0875 mm). One such piece of equipment is a step motor with a large number of poles. The step motor must be directly coupled to a printing support feeding roller. The feeding roller must be of the friction type that is able to assure sufficient contact surface between feeding device and the support being fed. Further it has to have a suitable friction coefficient. It has been experimentally determined that the use of different types of feeding mechanism is not suitable in providing such small increments of the support. Such small increments excludes pin tractors because of elastic deformation which takes place at the supporting edges, which is a large percentage of the shift. This is due to the friction between the fixed part of the printer and the printing support; in fact the local shift of the support in correspondence to the tractors does not cause sufficient local tension in the support necessary to overcome the starting friction.
Additionally the use of an economical step motor, with a small number of poles and therefore with a high angle of rotation between a stable position and the subsequent position is also excluded. Assuming that a typical pass angle for such a motor is 7.5.degree.; a direct coupling of such motors to the feeding roller would require the use of feeding rollers having a diameter of few millimeters. Therefore the use of reduction gears with a reduction ratio of about 1/15.div.1/30 is necessary. Such a ratio may be obtained with several reduction gears but introduces slack equal to the desired shift. Moreover such a ratio requires one reduction gear having special teeth, thus adding to the expense.
We will consider next the speed limits of the support shift because of the use of a step motor having a large number of poles or because of the use of reduction gears. Generally matrix serial printers are bidirectional; that is, they are able to print during passes from the left to right as well as from right to left. A pass generally requires less than 1 second to be executed. The advance of the support between a pass and the subsequent one must, therefore, be executed in the shortest possible time. For instance, the execution of a line feed in 100 msec time, affects the printer throughput by 10%. The currently used line spacing has a width of 1/6" or 1/8" (respectively 4.25 mm and 3.18 mm). If a support advances 1/288" (0.0875 mm) at each step motor pass, a line feed advance of 1/6" requires 48 motor passes. Likewise a lead of 1/8" requires 36 motor passes. It is known that the step motors have a limited speed range, of N passes per second, within which they may operate with an almost constant rated torque. Beyond a predetermined speed the torque rapidly decreases. It is therefore required that the line feed be performed with a smaller number of motor passes in order not to penalize the execution time of the line feed.
These disadvantages are overcome by the high quality printing method of the present invention, which may be used with fast and economical dot matrix printers where character printing is performed according to a convention 9.times.5 or 9.times.9 dot matrix.